Process of case hardening



Patented Feb. 17, 1931 PIA-TEN r OFFICE JOHN J. EGAN, OF B ROOKL YN, NEWYORK, ASSIGNOR TO ELECTR O METALLURGICAL COMPANY, A CORPORATION OF WESTVIRGINIA PROCESS OF CASE HARDENING No Drawing.

My invention relates to the production of nitrided layers or cases onferrous materials by heating the materials in gaseousnitrogencontaining'substances and especially to the production of hard,wear-resistant cases. The object .of my invention is to provide meansfor accelerating and intensifying the action of nitriding gases onferrous materials so as to reduce the time required to produce a case ofgiven hardness or thickness.

Ferrous metals are preferably nitrided at low temperatures in order toavoid deleteriously affecting the material of the objects to be casedand to avoid the formation of brittle cases. ,To enhance the productionof the case at low temperatures, critical amounts of various promotingagents, such as aluminum have been alloyed with the material to behardened. The presence of promoting agents in the objects to be hardenedpermits the use of low temperatures of the order of 460 C. to 580? C. Byheating such an al-' tetroxide or nitrogen pentoxide may be used as theaccelerator and mixtures of the oxides may also be used. The acceleratedaction resulting from the use of an oxide of nitrogen in conjunctionwith the case-producingnitriding agent may perhaps be due to themomentary production of nitrogen in an active form that readily combineswith the constituents of the ferrous material. The following equationrepresents a reaction producing elemental nitrogen which may Applicationfiled February 26, 1930. Serial No. 431,643.

take place in the heated nitriding chamber when ammonia is used as-thenitriding agent and nitric oxide is used as the accelerator The additionof nitrogen gas to ammonia does not accelerate the formation of thenitrogencontaining coating. However, the exact nature of the reactionsare unknown to me and the acceleration may be due to the formation ofsome unstable nitrogen compound not yet identified. I do not limit myinvention by theforegoing attempts to explain it.

Any substantial amount of the accelerator may be used. When a gaseousnitriding agent is .used, the preferred proportions of the nitridingagent and the accelerator are those corresponding to the chemicalreacting weights of the gases. Thus, a mixture consisting of 40% ammoniaand 60% nitric oxide by weight is given by way of example, but

other proportions of nitrogen oxides will acelerate the formation of anitrogen-containing case.

As illustrative of-myifivention, an alloy article containing 95.5% iron,0.90% aluminum and small amounts of silicon, manganese, carbon,phosphorus, sulphur, nickel, molybdenum and chromium was placed in aclosed container. The 'air in the container was displaced by a gasmixture consisting of 40% ammonia and 60% of nitric oxide by weight andthe container and its contents were heated to about 460 C. for fourhours whilst a current of the gas mixture was passed into the container.Upon cooling, the

article was found to have a thin, adherent,

wear-resisting, nitrogen-containing coating having a hardness greaterthan that corresponding to 1000 Brinell.

A similar alloy article was heated in ammonia alone for the same timeand at the same temperature but substantially no hardbe suggested by theforegoing description. For example, nitrates, such as lead nitrate,which give ofi nitric oxide when they are heated, may be placed in thecontainer with the material to be case-hardened, so that oxide ofnitrogen will be provided by the decomposition of the nitrate. Owing tothe wide range of modification of which this invention is capable,applicant does not wish to be understood as imposing limitations on theinvention, except such-as are specifically in dicated in the appendedclaims or, imposed by the prior art.

I claim:

1. The method of nitriding metal articles which comprises heating thearticles in the presence of a nitriding agent and at least one of theoxides of nitrogen. I

2. The method of nitriding metal articles which comprises heating thearticles in the presence of a nitriding gasand at least one of theoxides of nitrogen.

3. The method of nitriding metal articles which comprises heatingthearticles in'the presence of a nitriding gas and nitric oxide.

4. The method of nitriding metal articles which comprises heating thearticles in the presence of a gas comprising ammonia and at least one ofthe oxides of nitrogen.

5. The method of nitriding metal articles which comprises heating thearticles in the presence of a gas comprising ammonia and V nitric oxide.

6. The method of nitriding iron-containing articles which comprisesheating the articles in a mixture of gases containing a nitriding gasand at least one of the oxides of nitrogen in proportions about equal totheir chemicalreacting weights.

7. The method of nitriding iron-containing articles which comprisesheating the articles in a gas comprising ammonia and nitric oxide inproportions of about 4 parts by weight of ammonia and about 6 parts byweight of nitric oxide.

8. The method of nitriding iron alloy articles which comprises heatinthe articles to about 460 C. to about 580 in the presence of a gascomposed of 4 parts of ammonia by weight and about 6 parts by weight ofnitric oxide.

In testimony whereof, I aflix my signature.

- JOHN J. EGAN.

